Hello Chris,
If you study the users and users profile tables, you will see pretty easily how the data is structured by DNN. Basically, the users tables stored the basic details such as username, firstname and surname, IOW the fields that everyone has.
The user Profile table, can store any properties that you care to store, eg phone, modile, favourite hair colour, brand of motorcycle. The beautiful thing about the user profile is that you can expand or contract what we store about a person, based on your individual preferences.
If you take a look at the table named ProfilePropertyDefinition you will see that it defines a list of potential properties that you might store. You are free to add additional Properties into this list, eg "favourite Female Irish vocalist". Once that ProfilePropertyDefinition has been created, you can then add it into the list of properties stored in the UserProfile
Have a play with the following tables and it will be clear as mud :)
SELECT * FROM users
SELECT * FROM dbo.UserProfile
SELECT * FROM dbo.ProfilePropertyDefinition
Look at both the data and the columns in the table, and then try doing a few queries where you select you profile. You will quickly see how you can build your definition.
HTH